it might happen that by the time PID 1 adds our process to the scope unit the process might already have died, if the process is short-running (such as an invocation to /bin/true).
When systemd picked some recycled name for scope following error could appear:
'Failed to start transient scope unit: Unit run-XXXXX.scope already exists.'
Synchronously wait until the scope unit we create is started.
It should work now.